It is two weeks before the end of high school. Evan (Michael Cera) and Seth (Jonah Hill) are long-time friends in their senior year of high school, about to leave for different colleges. Their mutual friend, Fogell (Christopher Mintz-Plasse), has plans to acquire a fake ID, so Seth agrees to buy alcohol for a party that his crush, Jules (Emma Stone), is hosting. Evan also offers to buy Goldslick vodka for his long-time crush Becca (Martha MacIsaac). There is some tension between Seth and Evan because Evan and Fogell will both be going to Dartmouth while Seth will be going to a state college.
While purchasing the alcohol Fogell is assaulted by a hooded robber, and the police soon arrive. The two officers, Michaels (Seth Rogen) and Slater (Bill Hader), appear to be fooled by Fogell's fake ID (which has the single name "McLovin" as his identity) and offer Fogell a lift to the party. On the way, however, they make numerous stops and display many instances of inappropriate use of police powers, including stealing beers, drinking on the job, unnecessary use of police lights, and improper use of their firearms.
While the police officers are talking to Fogell at the liquor store, Evan and Seth make the assumption that Fogell has been arrested. Seth is suddenly hit by a car, but seems uninjured. To avoid being reported, the driver, Francis (Joe Lo Truglio), agrees to take them to a party, where they reason they can steal the alcohol they need. However, Francis is not welcome at the party and is subsequently assaulted by the host, Mark (Kevin Corrigan), and Seth and Evan are forced to leave, but not before smuggling out beer in laundry detergent bottles.
Once at a safe distance the two stop to catch their breath and Seth accuses Evan of bailing on him. Evan defends himself saying that the whole thing was Seth's idea. Seth retorts saying that not only did Evan bail on him at the party but also at earlier that morning when a bully spat on Seth, and will again be bailing on him when he goes to Dartmouth that Fall. Evan then angrily confesses that he feels Seth has been holding him back for the last three years and that while he could have been out making friends and chasing girls he sat around talking "bullshit." The two then get into a shoving match. During the fight, Seth is again hit by a car, this time the police cruiser occupied by Slater, Michaels, and Fogell. Officer Slater prepares to arrest the teens but Evan runs off and the two officers chase him while the reunited Seth and Fogell make off with the rest of the alcohol. The trio eventually make it to the party.